Small electronic hearing devices for being worn at an ear or within an ear canal of a user are becoming increasingly popular. Examples of such devices are earphones, for instance used in conjunction with personal audio/video players, gaming units and mobile phones, ear-level communication devices, active hearing protection devices, in-ear monitors as well as hearing aids, sometimes also referred to as hearing instruments or hearing prostheses. Such devices are available in a number of different styles depending on how they are worn, for instance as behind-the-ear (BTE), in the crest of the cymba, in-the-ear (ITE), in-the-canal (ITC), completely-in-canal (CIC) or hybrid BTE/ITE devices. In many applications it is preferred that the device is as inconspicuous as possible, e.g. for reasons of aesthetics and wearing comfort. This is frequently achieved by placing the device into the ear canal of the user, either partly or fully. Alternatively, the devices are designed to be small enough to fit into the crest of the cymba or to be worn entirely behind the pinna.
In order to provide an audio signal to the ear drum of the user the mentioned devices require a loudspeaker, often also referred to as a receiver, i.e. a unit that converts an electrical signal conveying an audio signal into acoustic energy in the form of sound waves (more generally referred to as an electro-acoustic transducer). Such receivers need to be very small, especially in order to fit into the ear canal of a person. Smaller receivers allow to design hearing devices which can be inserted deeper into the ear canal, e.g. into the bony portion, which provides the benefit of reduced occlusion effect. This particularly enables the design of deep-fitted CIC devices. Moreover, smaller receivers allow to design hearing devices that occlude the ear canal to a lesser extent, i.e. that leave the ear canal more open when the hearing device is inserted, providing the benefit of increased wearing comfort as well as a more natural sound perception. Especially open-fitted hybrid BTE/ITE devices, popularly referred to as receiver-in-canal (RIC) or receiver-in-the-ear (RITE) devices, benefit from such small receivers. Furthermore, also BTE devices and devices worn in the crest of the cymba profit from small receivers since it is mainly the size of the receiver that determines the degree of miniaturisation achievable for these devices.
Examples of such miniature receivers for hearing devices are disclosed in EP 0 851 710 A1, EP 1 209 948 A2, U.S. Pat. No. 5,960,093 and EP 0 548 580 A1. These receivers have a high maximum output power (MPO) at their mechanical resonance frequency, typically at around 2 to 3 kHz. Below this frequency the amplitude response of these receivers degrades by 5 to 10 dB/decade dependent on the resonance damping. Additionally, when used in an ITE hearing device with a vent, the vent increases the drop off to 40 dB/decade, when the vent not resistive, and otherwise a drop off 20 dB/decade results. Consequently, a common drawback of conventional hearing devices utilising these known miniature receivers is that the low-frequency sound components cannot be effectively reproduced.